WO 2005/106531 by the present Applicant, which is herein incorporated by reference for all purposes, discloses a method for providing image data which may be used to construct an image of an object based on a measured intensity of several diffraction patterns. This method is known as a pytochgraphical iterative engine (PIE). In PIE an iterative phase-retrieval method is used to determine an estimate of the absorption and phase-change caused by the object to a wave field as it passes through or is reflected by the object. This method uses redundancy in the plurality of diffraction patterns to determine the estimate.
WO 2010/064051 by the present Applicant, which is incorporated herein by reference for all purposes, discloses an enhanced PIE (ePIE) method wherein it is not necessary to know or estimate a probe function. Instead a process is disclosed in which the probe function is iteratively calculated step by step with a running estimate of the probe function being utilised to determine running estimates of an object function associated with a target object.
WO 2008/142360 by the present Applicant, which is herein incorporated by reference, discloses a method of three dimensional imaging. In this method, a complex wave field of illumination is determined at a plurality of planes downstream of a source of the illumination. Typically, these planes intersect an object. Using knowledge of the illumination at each plane, an iterative process such as that disclosed in WO 2005/106531 or WO 2010/064051 may be performed to construct an estimate of the object at that plane, from which an image of the object at that plane may be produced. By repeating the iterative process for each of the plurality of planes, a three dimensional image of the object may be obtained.
However, problems have been noted in that images produced by the method of WO 2008/142360 may be complicated by Fresnel-type fringes relating to a plane adjacent to the plane of interest, such as a plane in front (upstream) or behind (downstream) of the plane of interest. Furthermore, the method takes no account of multiple scattering effects. That is to say, it does not take into account the effect of the object on the illumination as it propagates through the object, such as due to scattering from upstream layers of the object previously traversed by the illumination.
It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.